This invention relates to a method and apparatus for controlling the concentricity of a coating applied to an optical fiber. The diameter can also be controlled.
During the process of drawing optical fibers, a protective coating is applied to the drawn fiber before the pristine surface of the fiber is damaged by subsequent handling, either during manufacture or subsequent use. This coating step is typically performed as an integral part of the drawing process to ensure that the coating material is applied before the surface of the fiber is damaged.
A coating material commonly used in the manufacture of optical fibers is an acrylate-based composition which is curable by exposure to ultraviolet (UV) light. This material is applied to the surface of the fiber in a liquid state and is subsequently exposed to UV light for curing. The coating material may be applied in one or more layers, with a two-layer coating system being a preferred embodiment. The primary coating is applied directly to the surface of the fiber, and the secondary coating is applied over the primary coating.
Higher draw rates reduce the cost of manufacturing optical fiber. When coating an optical fiber, it is important to produce, at high draw rates, coatings which have uniform diameter and which are concentric with the fiber. Both of these attributes contribute to ease in splicing and connectorization of the fiber, thereby providing for lower losses in an installed fiber application. Market demands continue to place more stringent tolerances on the diameter and concentricity of optical fiber coatings.
A fiber drawing and coating system, as currently used in the production of optical fibers, is shown in FIG. 1. Fiber 10 is drawn from preform 11 which is heated in furnace 1. Fiber 10 passes through fiber cooling device 2 and then through primary coater 3 where it is coated with a layer of primary coating material. The primary coating layer is cured in primary coating curing device 4, and the diameter of the fiber including the cured primary coating is measured by device 5. Curing device 4 typically comprises an irradiator array. Fiber 10 then passes through secondary coater 6 where it is coated with a layer of secondary coating material that is cured in curing device 7 which is similar to curing device 4. The diameter of the fiber including the cured secondary coating is measured by device 8. Tractor means 9 pulls the fiber from furnace 1 and through the intermediate devices. The drawn fiber is typically taken up onto spools by a winder (not shown) for further processing. Coating material is supplied to coaters 3 and 6 from sources 12 and 14, respectively. The inlet or "bulk" temperature of the coating material can be maintained at a desired value by devices 13 and 15, respectively, which are in communication with the coating delivery line. Examples of coating material heaters can be found in U.S. Pat. Nos. 4,073,974 and 4,622,242.
FIG. 2 shows a typical coating die assembly currently used in the process of coating optical fibers. Fiber 21 enters coating die assembly 20 through guide die 22. Coating material is delivered to coating die assembly 20 through holes 24 in insert 23. The temperature controlled coating material is radially distributed about insert 23 before entering die assembly 20. The coating material is typically supplied to die assembly 20 under pressure. A pressurized coater insures that the level of coating material inside die assembly 20 is maintained at the same level throughout the fiber coating process. Fiber 21 exits coating die assembly 20 through sizing die 25. As fiber 21 passes through coating die assembly 20, the coating material is accelerated. As the coating material and fiber 21 enter sizing die 25, a portion of the coating material is pulled out with the fiber. The coating material that is accelerated by the fiber, but not pulled out with the fiber, recirculates within coating die assembly 20. Coating die assembly 20, as shown in FIG. 2, is similar to that disclosed in U.S. Pat. No. 4,531,959, the relevant portions of which are incorporated herein by reference. The coating may be applied using the method disclosed in U.S. Pat. No. 4,792,347 for reducing the formation of bubbles in the coating.
The amount of coating material which is drawn out with fiber 21 is dependent on the velocity profile of the coating material within sizing die 25. This velocity profile is most affected by the speed at which fiber 21 is drawn through coating die assembly 20, the geometry of sizing die 25 and the viscosity profile of the coating material in sizing die 25. The viscosity profile of the coating material is a function of its temperature, which is influenced by: i) the temperature of fiber 21; ii) the temperature of the walls of sizing die 25; iii) internal heat generation known as "viscous heating" which is the result of the conversion of mechanical energy to thermal energy via fluid friction; iv) the temperature of the incoming coating liquid; and v) the temperature of any surface with which the coating liquid thermally communicates. When the region in which the viscosity profile is controlled, by controlling the temperature of the coating material, is localized to land region 26 of sizing die 25, very responsive control of coated fiber diameter can be achieved. Die land region 26 is defined as the region at the exit of sizing die 25 where the diameter of inner wall 27 of sizing die 25 is substantially constant with distance from the exit of sizing die 25. U.S. Pat. No. 5,366,527, which is incorporated herein by reference, provides further discussion of the dynamics of the coating process.
U.S. Pat. No. 5,366,527 discloses a method for controlling the diameter of a coated optical fiber by heating the coating material in the coater; it teaches that the heating of the coating material should be localized to a region of the coating die assembly in which more rapid changes in the temperature of the coating material within the sizing die can be achieved. For example, heating may be advantageously localized in the portion of the sizing die surrounding land region 26. One embodiment of the invention disclosed in U.S. Pat. No. 5,366,527 is shown in FIG. 3 which shows a coating die assembly similar to that of FIG. 2. Temperature control jacket 30 is placed around the outside wall 31 of sizing die 25. Jacket 30 is capable of raising or lowering the temperature of the outside wall 31 of sizing die 25, thereby raising or lowering the temperature of inner wall 27. The temperature adjustment provided by jacket 30 can be controlled by a control system as shown in FIG. 1. If jacket 30 consisted of a resistance heater, for example, the coating diameter signal from device 8 would be processed to generate a control signal that varied the voltage applied to that resistance heater. The measurement of the diameter of the coated fiber determines the level of heating required to maintain the diameter of the coated fiber at a target value.
Very rapid changes in the temperature of the coating material can be achieved by adjusting the temperature of bottom surface 33 of sizing die 25, resulting in very responsive control of coated fiber diameter. In this regard, U.S. Pat. No. 5,366,527 teaches the use of temperature controlled chips or plates in thermal contact with the bottom of the sizing die. For example, a thermoelectric chip that employs the Peltier effect can operate as a heat pump to uniformly heat or cool that region of the sizing die from which the coated fiber exits. In another embodiment, a disk made of a high thermal conductivity material thermally communicates with the bottom of the sizing die. A heat transfer tube that is in thermal contact with the disk extends through both a resistance heater and a fluid-cooled heat sink, whereby the disk can uniformly heat or cool an annular region at the bottom of the sizing die.
While the method of U.S. Pat. No. 5,366,527 can control the coated fiber diameter in response to a signal that is generated in response to the deviation of the actual diameter with respect to a predetermined setpoint diameter, that apparatus is incapable of correcting for coatings that are not concentric with the optical fiber.
U.S. Pat. Nos. 4,321,072, 4,363,827, 4,390,897, 4,957,526 and 5,176,731 teach methods of applying coatings that are concentric with respect to an optical fiber. The concentricity of the coating with respect to the fiber is analyzed by a known technique such as one disclosed in those patents or in the publications: The Bell System Technical Journal, 55, No. 10 (December 1976), pp. 1525-1537; Applied Optics, 16 (1977), page 2383; or Advanced Instrumentation, 36 (1980), pp. 229-231. All of these patents teach that concentricity of the coating can be controlled in response to the error signal by mechanically shifting to coater to center the fiber in the coating. U.S. Pat. No. 4,363,827 states that the centering of the fiber within the coating apparatus can also be obtained by movement of the fiber. The systems disclosed in these patents are disadvantageous because of the complex mechanical devices that are needed to correct for fiber-coating non-concentricity.